Factors affecting coal particle ignition under oxyfuel combustion atmospheres

A set of 13 coals of different rank has been tested for ignition propensity in a 20-L explosion chamber simulating oxyfuel combustion gas conditions. Their char residues were also analysed thermogravimetrically. The effects of coal type, coal concentration (from 100 to 600 g/m3), O2 in CO2 atmospheres (up to 40% v/v) and particle size were investigated. The higher rank coals were significantly more difficult to ignite and mostly required higher energy chemical igniters (1000 or 2500 J) whereas the lower rank coals could be ignited with a 500 J igniter even at low coal dust concentrations. The minimum explosibility limit/ignition concentration in air varied slightly around a value of 200 g/m3, a little higher for low volatile coals and a little lower for high volatile coals. The ignition limit changed significantly, however, with O2 concentration in CO2, where coals required more oxygen to ignite. Most coals failed to ignite at all in 21% v/v O2 in CO2, but an increase to 30 or 35% v/v O2 gave ignition patterns similar to those in air. In addition, the minimum ignition concentration decreased with increase in O2. However, a further increase to 40% v/v O2 did not generally affect the minimum ignition concentration. Particle size had a non-linear effect on coal ignition. The fine particles (<53 ?m) behaved almost identical to the whole coal. However, the larger size fraction (>53 ?m) was generally more difficult to ignite and exhibited a much lower weight loss.